The Tasmanian montane and rainforest conifer genus Athrotaxis provides a system for investigating the relationship between leaf form and function and its adaptive significance. The two species differ markedly in leaf size, shape, degree of imbricacy and stomatal distribution, whereas natural hybrid swarms and glasshouse-grown hybrid progeny are highly variable for these traits. In glasshouse-grown plants of the true species and a diverse hybrid progeny, stomatal conductance and density were strongly correlated, and varied by approximately 400% among individuals. Hybrids displayed lower stomatal densities and less discrimination of ¹³C than the true species, leading to a negative relationship between stomatal density and δ¹³C. In contrast with the highly variable stomatal densities and δ¹³C in glasshouse plants, field-grown plants were highly conservative in both characters. This, combined with relatively low stomatal density and high water-use efficiency in field-grown plants suggests optimisation of the trade-off between assimilation and water loss. Foliar conductance in the light for the hybrids and A. selaginoides was only 4–6 times as great as, and was strongly correlated with, conductance in the dark, suggesting incomplete stomatal closure or high cuticular conductance. Athrotaxis cupressoides was less ‘leaky’. This may reflect adaptation to its more exposed habitat.